Author: Richard Martin <martin@...> Time: Fri Sep 13, 2002 6:03 am PDT Link
Dear Norman
I have an RV8 that is equipped with the Sam James Cowl aka NACA cowl, or Barnard Holy Cowl, and plenum. The inlet height is fixed with this type of cowl and ordinarily would be difficult to change. The good news is that there is no reason to change it.Will your airplane be faster? Absolutely. With normal care and workmanship during the installation, you should gain 7 to 10 mph. With extra care in the installation, such as ducting the air from the inlet ring to the plenum, and making sure that the plenum is well sealed all the way around, 10 to 15 mph can be achieved. The standard inlet rings for the NACA cowl have a 4 3/4" ID. This will provide more than enough cooling even at temperatures of 100 degrees plus. (A Positech cooler will not be adequate with a 180 or 200 hp engine, buy at least a 9 row Niagara or Stewart Warner cooler). I now have 570 hours on my RV8 with a IO360AiB6 engine pumped up to 220 hp. I have had no problems with the cowl other than possibly bein!
g a little to cool during the winter. I have solved this by designing 4 smaller size inlet rings for the NACA cowl. The best all around size is 4" ID. It works well between temperatures 50 to 90 f.
There is a plus with the smaller rings and that is they further reduce cooling drag and the 4" ring will average a 4 knot increase in speed. Another benefit of the NACA cowl is an average boost in manifold pressure at cruise of 2 inches. I hope this info helps you feel more comfortable with your cowling selection.
Dick Martin
RV8 N233M
the fast one
>
> Kevin, nice informative post. I am planning on using the Sam James cowl with
> the smaller round inlets and plenum. Do you think inlet hieght will not be a
> factor and do you think that cowl will make the airplane faster?
>
> Thanks,
> Norman Hunger
> Do not archive questions
>
> > --> RV8-List message posted by: Kevin Horton <khorto1537@r...>
> >
> > I went to the EAA Chapter 486 RV Fly-in at Fulton, NY (FZY) last
> > weekend. I spent quite a bit of time looking at cowlings, as I have
> > just started working on mine. I noted one thing that is worth
> > thinking about if you want to reduce drag.
> >
> > I'm building an RV-8. Van specifies the height of the air inlets as
> > 2 5/8 to 2 3/4 inches (IIRC). I didn't have a ruler with me, but it
> > was quite clear that many RVs have air inlets that are quite a bit
> > larger than called for on the RV-8 plans. These RVs also had quite a
> > large flat area extending above and/or below the spinner, which also
> > supports my theory that their cowl halves weren't trimmed as much at
> > the front as Van intended.
> >
> > So what, you ask? Well, if the height of the cooling air inlets is
> > increased, the amount of air that goes into the cowling increases.
> > If we look at the aircraft as our frame of reference, all this air
> > has to slow down to pretty close to zero speed. Or, looking at the
> > earth as our frame of reference, all this air has to be accelerated
> > from a stop to the speed of the RV. This acceleration requires a
> > force to be exerted on the air by the aircraft, and this force is
> > felt by the aircraft as drag.
> >
> > I did a very rough back of the envelope calculation, and if the air
> > inlet height on both sides is increased by 1/2 inch, that gives about
> > a 4% drag increase, and would cause a speed decrease of about 2 kt
> > for a 160 hp RV, and about 2.25 kt for a 200 hp RV. If the inlet
> > height is one inch too high, you can about double those numbers.
> >
> > This calculation ignores the drag of the flat part of the cowl that
> > may be exposed above and below the spinner if the front part of the
> > cowl has more height than Van intended. So, the real effect could be
> > greater than this back of the envelope calculation would suggest.
> >
> > Caveat - this calculation used quite a few simplifications. Total
> > drag of a nominal aircraft was calculated based on Van's 75% cruise
> > numbers, and an assumed 79% prop efficiency (obtained from a Hartzell
> > prop efficiency map). Drag increase calculation assumed that the air
> > entering the cowling was at free stream ambient conditions, and that
> > the air was slowed to 10% of its initial velocity (relative to the
> > aircraft).
> >
> > So, if you want to get Van's performance numbers, be sure to pay
> > attention to the height of the air inlets on the cowl.
> >
> > Take care,
> > --
> > Kevin Horton RV-8 (spinner, cowling)
> > Ottawa, Canada
> > http://members.rogers.com/khorton/rv8.html
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